A conventional power supply includes at least a main power system, a standby power system, a transformer and a conversion output unit at the secondary side of the transformer. The main power system and the standby power system have respectively a large capacitor to adjust energy output and storing. When the power supply is started it performs charging rapidly and at the same time provides sufficient power output to the main power system and the standby power system. However, the voltage of the capacitor rises according to an exponential curve. Hence after the power supply starts there is a charging period in which the capacitor does not reach the voltage level of system regular operating condition. Based on the formula P=V×I (Power=Voltage×Current), when the power supply starts initially the capacitor is not yet being charged, the voltage is low but the secondary side of the transformer has already supplied full load output power. Hence current is very high, and an inrush current occurs as shown in FIG. 1. Such a condition likes short circuit. If the power supply starts at the peak voltage value of input AC potential the phenomenon of inrush current is even more serious. To suppress the inrush current a conventional approach employs a thermal resistor of a negative temperature coefficient (or NTC in short) to provide a greater resistance at the start time to harness the current. The thermal resistor has its resistance gradually decreased as the temperature increases while the power supply is in duty. Hence impact to the regular operating current can be reduced. But at a regular room temperature if the resistance of the thermal resistor is too high or the temperature is too low the resistance rises excessively, the power supply cannot be started normally. In the regular room temperature the resistance is too low and the efficacy of suppressing the inrush current is not desirable. Moreover, the thermal resistor continuously consumes power and generates heat during regular operation of the power supply. As a result total efficiency of the product decreases. And the unnecessary heat also affects product life span. Although the power supply usually has a relay to short circuit the thermal resistor during regular operation, adding the relay increases the cost. Thus such a practice also has its share of disadvantage.